1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to an in-plane switching mode liquid crystal display device and a fabrication method thereof capable of preventing sealant leakage.
2. Description of the Related Art
Generally, an LCD device is a display device in which data signals according to image information are individually supplied to liquid crystal cells arranged in a matrix form. Light transmittance of the liquid crystal cells is controlled to display a desired image. The LCD device includes a liquid crystal display panel with pixel-unit liquid crystal cells arranged in a matrix form and a driver integrated circuit (IC) for driving the liquid crystal cells. The liquid crystal display panel includes a color filter substrate and a thin film transistor array substrate, which face each other, and a liquid crystal layer filled in between the color filter substrate and the thin film transistor array substrate.
On the thin film transistor array substrate, data lines for transmitting data signals supplied from a data driver IC to the liquid crystal cells and gate lines for transmitting scan signals supplied from a gate driver IC to the liquid crystal cells cross one another. Liquid crystal cells are defined adjacent to the crossings of the data and gate lines. A data pad and a gate pad are provided at one end portion of each data line and gate line, respectively. A data signal is applied to a liquid crystal cell through a data line from the data driver IC. A scan signal is applied to a liquid crystal cell through a gate line from the gate driver IC. The gate driver IC sequentially supplies scan signals to the gate lines to sequentially select liquid crystal cells arranged in a matrix line by line. Data signals are supplied from the data driver IC to the liquid crystal cells of the selected line.
A common electrode and a pixel electrode having a finger structure are formed in each pixel, to apply a lateral electric field. By controlling a voltage applied to the common electrode and the pixel electrode, the light transmission ratio of each of the liquid crystal cells can be individually controlled. A thin film transistor is formed in each liquid crystal cell and used as a switching device. When a scan signal is supplied to the gate electrode of the thin film transistor through the gate line, a conductive channel is formed between the source electrode and the drain electrode. At this time, a data signal is supplied to the source electrode of the thin film transistor through the data line such that the data signal is supplied to the pixel electrode by way of the drain electrode. Accordingly, an electric field is applied to the liquid crystal layer of the corresponding liquid crystal cell.
FIG. 1 is a plan view showing a liquid crystal display panel having a thin film transistor array substrate and a color filter substrate attached in a facing manner in a general in-plane switching mode LCD device. As shown in FIG. 1, the liquid crystal display panel 100 includes an array region 113 in which liquid crystal cells are arranged in a matrix form; gate pads 114 formed at the end of the gate lines 108; and data pads 115 formed at the end of the data lines 109. The gate pads 114 and the data pads 115 are formed at a peripheral region of a lower substrate 101, which is not overlapped with the upper substrate 102. Each of the gate pads 114 supplies a gate signal from the gate driver IC to the gate lines 108 of the array region 113. Each of the data pads 115 supplies a data signal from the data driver IC to the data lines 109 of the array region 113.
As shown in FIG. 1, data lines 109, to which image information is applied, and gate lines 108, to which a gate signal is applied, are arranged to cross each other on the lower substrate 101 of the array region 113. Adjacent to each of the crossings, there is a thin film transistor for switching a liquid crystal cell, a common electrode and a pixel electrode connected to the thin film transistor to drive a liquid crystal cell. Liquid crystal (not shown) is filled into a cell gap between the upper substrate 102 and the lower substrate 101. The liquid crystal is held in place by a sealant 116 formed along an outer edge of the array region 113.
In the upper substrate 102 of the array region 113, color filters are formed, which are separated into cell regions by a black matrix 103 to prevent light leakage. The thin film transistors (not shown), the gate lines 108, the data lines 109, a part of each of the gate pads 114 and a part of each of the data pads 115 of the lower substrate are also covered by the black matrix 103. In FIG. 1, only a black matrix 103 formed at an outer edge of the array region 113 is shown. A metal material, such as Cr, can be used as the black matrix 103. However, because a metal material can affect the lateral electric field between the pixel electrode and the common electrode, a resin black matrix is preferably used.
FIG. 2 is a sectional view along line II-II in the liquid crystal display panel of in FIG. 1. As shown in FIG. 2, liquid crystal 139 is filled in between the upper substrate 102 and the lower substrate 101. The liquid crystal 139 is held in place by sealant 116 along the periphery of the upper substrate 102 and the lower substrate 101. A specified cell gap is maintained by a spacer 105 disposed within the liquid crystal 139. The lower substrate 101 includes a transparent substrate 131, a gate pad 114 to which a scan signal is applied from the gate driver IC, a thin film transistor (TFT), a common electrode 133 and a pixel electrode 137.
The TFT includes a gate electrode 132, source electrode 137a, a drain electrode 137b and a semiconductor layer 135. A gate insulating layer 134 is formed between the gate electrode 132 and the semiconductor layer 135. An ohmic contact layer 136 is formed between the semiconductor layer 135 and the source and drain electrodes 137a and 137b. 
Also, in the array region, the common electrode 133, which is formed when the gate electrode 132 is formed, and the pixel electrode 137, which is formed when the source/drain electrodes 137a and 137b are formed, are spaced apart at a predetermined interval across the lower substrate 101. A passivation layer 138 is formed over the entire surface of the thin film transistor (TFT) and the array region.
Referring to the upper substrate 102, the black matrix 103 and the color filter 122 are formed on a transparent substrate 121. An overcoat layer 123 for planarization is formed on the black matrix 103 and the color filter 122. In the case of a small-sized panel such as in a notebook computer, the black matrix 103 is formed at an outer edge of the upper substrate 102 and overlaps the sealant 116. However, because the black matrix 103 is made of the resin, its adhesion to the substrate 121 is weak such that the black matrix 103 will come off the substrate 121 after frequent shaking of the substrate. Thus, liquid crystal may leak because of a crack that may subsequently occur in the overcoat layer 123 or the sealant 116.
FIG. 3 is a sectional view of another related art LCD device. As shown in FIG. 3, in the case of forming the black matrix of Cr, the adhesive strength between the sealant 116 and the black matrix 103a is improved. However, the Cr black matrix affects the electric field between the pixel electrode 137 and the common electrode 133 such that cross-talk can occur, which degrades the picture rendering capability of the liquid crystal display panel.